Compared to existing alternatives, only ProtonDrive comes to mind as something that could work for encrypted cloud backups. Backblaze has a longer history in storage and has a strong track record of keeping data safe. They also do not necessarily hold the private key as stated here:
You have the option with Backblaze to add an additional layer of privacy via a user-selected passphrase. This passphrase will be used to encrypt your private key. This passphrase is your responsibility to remember and safeguard. This is important: if you forget or lose this passphrase there is no way that anyone, including Backblaze, can decrypt, and thus restore, your data.